You who are a fan of Deskmod and complete system customization, can lose hope of replacing Windows 7 boot screen. In a statement, Karen Wong (responsible for Microsoft's Core User Experience team) said:

The reasons are clear, that is, we cannot guarantee the safety of a system that allows the execution of elements for memory right on boot. At the start of the operating system boot, it needs to be closed and run in "safe mode" because, for example, antivirus and firewall are not fully functioning.

I think you will have to conform to the themes and wallpapers made available by Microsoft itself. 😛

Source: Wintech